discharge of the battery, but a second point of contact immediately produces them. It is said that a painful sensation may be produced by a discharge conveyed through the medium of a stream of water. The electric currents created in these fishes exercise all the other known powers of electricity: they render the needle magnetic, decompose chemical compounds, and emit the spark. The dorsal surface of the electric organ is positive, the ventral surface negative. [The literature on the electric organ of Torpedo is very extensive. Here may be mentioned _Lorenzini_, “Osservazioni intorno alle Torpedini,” (1678); _Walsh_, “On the Electric Property of the Torpedo,” in Philos. Trans., 1773; _Hunter_, “Anatomical Observations on the Torpedo,” _ibid._; _Davy_, “Observations on the Torpedo,” in Philos. Trans., 1834; _Matteucci_ and _Savi_, “Traité des Phénomènes Electro-Physiologiques,” 1844.] Of the genus _Torpedo_ six species are known, distributed over the Atlantic and Indian Oceans; three of them are rather common in the Mediterranean, and one (_T. hebetans_) reaches the south coast of England. They attain to a width of from two to three feet, and specimens of that size are able to disable by a single discharge a full-grown man, and, therefore, may prove dangerous to bathing persons. Other genera, differing from _Torpedo_ in the position and structure of some of the fins, are found in other tropical and sub-tropical seas, viz. _Narcine_, _Hypnos_, _Discopyge_ (Peru), _Astrape_, and _Temera_. All, like electric fishes generally, have a naked body. A large fish, of the general appearance of a Torpedo, has been found at Monte Bolca; and _Cyclobatis_, from the upper cretaceous limestone of Lebanon, is probably another extinct representative of this family. FOURTH FAMILY--RAJIDÆ. _Disk broad, rhombic, generally with asperities or spines; tail with a longitudinal fold on each side. The pectoral fins extend to the snout. No electric organ; no serrated caudal spine._ RAJA.--Two dorsal fins on the tail, without spine; tail with a rudimentary caudal fin, or without caudal. Each ventral fin divided into two by a deep notch. Teeth small, obtuse, or pointed. Pectoral fins not extending forwards to the extremity of the snout. Nasal valves separated in the middle, where they are without a free margin (see Fig. 1, p. 34). [Illustration: Fig. 126.--Raja lemprieri, from Tasmania.] [Illustration: Fig. 127.--Dermal spines of a male Thornback, Raja clavata.]